The soil will cool faster because it has a lower specific heat than water (even if it is wet). The specific heat is a thermodynamic property. It is the quantity of energy required to raise a temperature by one degree Celcius (or Kelvin). By this property, you can measure how much energy a material need to raise its temperature (or how much energy it needs to lose by decreasing the temperature).
The specific heat of water is 1.001 Btu(IT)/(lbm °F).
The specific heat of a dry soil is 0.19 Btu(IT)/(lbm °F). The specific heat of a wet soil is 0.35 Btu(IT)/(lbm °F).
